1. Field of the Invention
The present invention relates to a photosensitive resin composition and a color filter provided with a protective layer formed of the photosensitive resin composition, and more particularly to a color filter which can realize color liquid-crystal displays having excellent display quality and high reliability. The present invention also relates to a novel epoxy-containing copolymer resin usable in the photosensitive resin composition.
2. Background Art
In recent years, color liquid-crystal displays have drawn attention as flat displays. They generally have a structure comprising: a color filter; a counter electrode substrate facing the color filter; and a liquid-crystal layer provided in a gap (1 to 10 xcexcm) between the color filter and the counter electrode substrate. The structure of the color filter is such that a colored layer of a black matrix and a plurality of colors (generally three primary colors, red (R), green (G), and blue (B)) or, in recent years, a color hologram is provided on a transparent substrate and a protective layer and a transparent electrode are stacked thereon in that order. These color liquid-crystal displays are constructed so that a color image is obtained by controlling the light transmission of the liquid-crystal layer in its portions corresponding to pixels of respective colored layers R, G, and B or a color hologram.
In the color filter, the protective layer, when the color filter comprises the colored layer, functions to protect and flatten the color filter. In general, color liquid-crystal displays have a problem that the presence of uneven gap attributable to the waviness of the surface of the transparent substrate in the color filter, uneven gap among R, G, and B pixels, or uneven gap within pixels of R, G, and B lowers the flatness of the transparent electrode. This causes uneven colors and uneven contrast, leading to lowered image quality. In particular, in color liquid crystal displays of STN (super-twisted nematic) system, the flatness greatly affects the image quality. This renders flattening by the protective layer very important.
In use, the color filter is adhered to the counter electrode. The assembly is then tested for display quality. In consideration of reutilizability of the color filter when the assembly is judged to be unacceptable in the display quality test, the protective layer is preferably provided only in specific regions so as to cover the colored layers on the transparent substrate. To this end, the protective layer has been formed using a photocurable resin which permits portions to be cured to be easily limited through a mask.
In the formation of the protective layer using the conventional resin, an organic solvent is used in the development after exposure. This is troublesome in handleability and wastewater treatment and further lacks in profitability and stability. In order to solve this problem, a photocurable resin has been developed which, by virtue of the introduction of an acidic group into the photocurable resin, permits alkali development after the exposure.
In this type of photosensitive resin composition, after patterning by ultraviolet irradiation, development with an alkali is carried out, followed by heat treatment to prepare a color filter. In the development with an alkali, patterning is carried out through the action of carboxyl groups in the copolymer resin. After the development, water resistance and alkali resistance are required of the copolymer resin. Therefore, the carboxyl group remaining in the photocured pattern should be heat treated in the presence of an epoxy resin or the like to impart water resistance and pattern strength.
For this reason, the present inventors have proposed, in Japanese Patent Application No. 99240/1998, a copolymer resin which, as an alkali-developable photocurable copolymer resin, enables alkali-soluble groups, such as carboxylic acid groups, and radically polymerizable groups specifying curability, such as acryloyl group, to be regulated by taking into consideration curability, alkali solubility and the like. Further, in Japanese Patent Application No. 216868/1998, the present inventors have proposed a photosensitive resin composition comprising this alkali-developable photocurable copolymer-resin and an epoxy resin, such as a cresol novolak type epoxy resin, and a color filter. However, the provision of a photosensitive resin composition possessing excellent curability, alkali resistance and other properties and a color filter, which can be produced by a simple process, has excellent flatness, and can realize highly reliable color liquid-crystal displays having excellent display quality, have been desired in the art.
Accordingly, it is an object of the present invention to provide a photosensitive resin composition possessing excellent hot pure water resistance, solvent resistance, heat resistance, alkali resistance, sensitivity, and pencil hardness and to provide a color filter which can be produced by a simple process, has excellent flatness, and exhibits excellent display quality and reliability.
It is another object of the present invention to provide a novel epoxy-containing copolymer resin usable in the photosensitive resin composition.
In order to attain the above objects of the present invention, according to one aspect of the present invention, there is provided an epoxy-containing copolymer resin comprising 0 to 55% by mole of constituent units represented by formula (1), 5 to 95% by mole of constituent units represented by formula (2), and 5 to 95% by mole of constituent units represented by formula (3), the constituent units represented by formula (1) having been partially reacted, through carboxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, the constituent units represented by formula (2) having been partially reacted, through hydroxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, said copolymer resin containing 5 to 95% by mole of (meth)acryloyl groups and having an acid value of 0 to 400 mg KOH/g and a weight average molecular weight of 5,000 to 1,000,000 as determined using polystyrene as a standard: 
wherein R represents hydrogen or an alkyl group having 1 to 5 carbon atoms; R1 represents an alkylene group having 2 to 4 carbon atoms; R2 represents an alkylene group having 1 to 4 carbon atoms; Z represents a 1,2-epoxy, 1,3-epoxy, or 1,4-epoxy group; and a, b, and c are integers respectively corresponding to said molar percentages.
Preferably, the copolymer resin further comprises 0 to 75% by mole of constituent units represented by formula (4) and 0 to 75% by mole of constituent units represented by formula (5): 
wherein R represents hydrogen or an alkyl group having 1 to 5 carbon atoms, R3 represents an aromatic carbon ring, R4 represents an alkyl or aralkyl group, and d and e are integers corresponding respectively to contents in terms of % by mole of the constituent units.
According to another aspect of the present invention, there is provided a photosensitive resin composition comprising: the above epoxy-containing copolymer resin; an alkali-developable photocurable copolymer resin; a bifunctional or higher polyfunctional photopolymerizable acrylate monomer; and an initiator,
the alkali-developable photocurable copolymer resin comprising 5 to 55% by mole of constituent units represented by formula (1) and 5 to 95% by mole of constituent units represented by formula (2), the constituent units represented by formula (1) having been partially reacted, through carboxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, the constituent units represented by formula (2) having been partially reacted, through hydroxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, the copolymer resin containing 5 to 95% by mole of (meth)acryloyl groups and having an acid value of 5 to 400 mg KOH/g and a weight average molecular weight of 5,000 to 1,000,000 as determined using polystyrene as a standard.
In the photosensitive resin composition, preferably, the alkali-developable photocurable copolymer resin and the epoxy-containing copolymer resin each further comprise 0 to 75% by mole of constituent units represented by formula (4) and 0 to 75% by mole of constituent units represented by formula (5).
According to a further aspect of the present invention, there is provided a color filter comprising: a transparent substrate;. a colored layer provided on the transparent substrate; and a protective layer provided so as to cover the colored layer, the protective layer having been formed by coating the above photosensitive resin composition, exposing the coating, developing the exposed coating with an alkali, and heating the developed coating.
As compared with conventional phenol novolak type epoxy resins and the like, the epoxy-containing copolymer resin according to the present invention has higher miscibility with an alkali-developable photocurable copolymer resin by virtue of the similarity of the structure, and, when used in a protective layer of a color filter, upon crosslinking at the time of heat curing, can form a pattern possessing excellent strength, alkali resistance, and water resistance.
The photosensitive resin composition of the present invention, by virtue of the use of an alkali-developable photocurable copolymer resin having controlled alkali solubility and curability and an epoxy-containing copolymer resin possessing excellent miscibility with the alkali-developable photocurable copolymer resin, possesses excellent hot pure water resistance, solvent resistance, heat resistance, alkali resistance, sensitivity, and pencil hardness. The use of this photosensitive resin composition in the formation of a protective layer can realize a color filter which can be produced by a simple process, has excellent flatness, and exhibits excellent display quality and reliability.